The Swedish Council on Technology Assessment in Health Care launched a project group in 1999 to systematically review and evaluate the existing literature on different caries-preventive methods. The aim of this article was to report the findings concerning the caries-preventive effect of fluoride mouthrinses (FMRs) in various age groups, with special reference to background fluorides. A systematic search in electronic databases for literature published between 1966 and August 2003 was conducted with the inclusion criteria of a randomized or controlled clinical trial, at least 2 years' follow-up, and caries increment in the permanent dentition (DeltaDMFS/T) as endpoint. Out of 174 articles originally identified, 62 met the inclusion criteria. These studies were assessed independently by at least two reviewers and scored A-C according to predetermined criteria for methodology and performance. The measure of effect was the prevented fraction (PF) expressed as percent. The level of evidence was based on 25 articles. The results revealed limited evidence (evidence level 3) for the caries-preventive effect (PF 29%) of daily or weekly sodium fluoride rinses compared with placebo in permanent teeth of schoolchildren and adolescents with no additional fluoride exposure and for a caries-preventive effect on root caries in older adults. Inconclusive evidence (evidence level 4) was found regarding the effect of FMRs in schoolchildren and adolescents exposed to additional fluoride sources such as daily use of fluoride toothpaste. No firm support for the use of FMRs was disclosed in a small number of studies designed for patients at caries risk. Furthermore, no association between the frequency of the rinses and prevented fraction or saved surfaces per year was found. In conclusion, this systematic review suggests that sodium fluoride mouthrinses may have an anti-caries effect in children with limited background of fluoride exposure, while its additional effect in children with daily use of fluoride toothpaste could be questioned. The need for further clinical trials to elucidate the effect of FMRs in risk patients and older adults is emphasized.